Musicians often use electronic effects to modify the sound of their instrument during composition and recording and during live performances. For many of these effects it is desirable for the musician to be able to control an aspect of the effect and this is typically achieved by interaction with controllers such as linear sliders, also known as faders, rotary knobs or foot pedals, also known as expression pedals. These controllers permit a musical effect to be varied continuously between a maximum and a minimum of that effect.
Using the example of the expression pedal, the effect of the expression pedal on the musical sound varies as the musician presses the pedal. The pedal is thus the controller that the musician uses to apply the desired amount of the effect to the sound. Different expression pedals exist such as volume pedals where the sound level is modified, wah-wah pedals where the frequency characteristics of the sound is modified and generic controller pedals where a control signal is used, for example, to adjust a parameter of a synthesizer. All of these pedals have in common the need to know how much the pedal has been pressed by the musician so that this can be converted into application of the correct amount of the effect being controlled by the pedal.
Other controllers, such as faders and knobs, use a similar principle to that described for the expression pedal. For a fader, the amount of the effect to be applied to the sound is determined by how much the fader has been moved. For a knob, the amount of the effect to be applied to the sound is determined by how far the knob has been rotated.
It is also possible to control a musical effect by using a controller which does not require the musician to touch it. For example, non-contact optical controllers can detect the proximity of an object such as the musician's hand as is described in U.S. Pat. No. 6,153,822 or foot as is described in U.S. Publication No. 2006/0278068 A1.
All of these controllers have in common the fact that the control of the effect is via the separation of a fixed reference element and a moveable element (such as pedal, slider, knob, ring), the position of which is chosen by the musician to achieve the desired musical effect.
The control of the musical effect can be accomplished in two ways: direct and indirect. Direct control of the effect is where the movement of the moveable element causes direct variation of a circuit element in an electronic circuit. Direct control therefore requires a circuit element whose electrical response can be varied by the movement of said moveable element. Indirect control of the effect is where the position of said moveable element with respect to said fixed reference element is measured to give a value of the mutual separation between said elements and this mutual separation value is used to control a separate circuit element in an electronic circuit or is used as an input parameter to a digital signal processing algorithm. It is therefore a requirement for indirect control to measure accurately the mutual separation of said fixed reference element and said moveable element.
Volume control pedals and wah-wah pedals have characteristically used potentiometers as a direct means of controlling the sound effect, such as is described in U.S. Pat. No. 3,530,224. A significant problem associated with this use of potentiometers is that most potentiometers are not designed for the large number of operating cycles required in such controllers, and those that are very expensive. As potentiometers wear out, the reproducibility of the control is compromised and noise can be introduced into the corresponding electronic circuits. Indeed it is common to need to replace the potentiometers in wah-wah and volume control pedals when they wear out. It is therefore highly desirable to have an alternative solution that is robust and does not suffer from wear during operation.
Optical position sensing can be used as an alternative to potentiometers, for example as is described in U.S. Pat. No. 6,859,541 B1, but the performance of such systems is vulnerable to degradation by contamination and they need to be cleaned to retain optimal performance. Moreover, they can contain delicate optical elements such as shades or films with graduated transparency which make then sensitive to shock and vibration with a corresponding reduction in long-term reliability. They can also be more expensive than potentiometers. It is therefore desirable to have an alternative position sensor that is unaffected by dirt and moisture contamination and is robust to shock and vibration such as is experienced during normal use.
Magnetic sensors such as Hall probes where a permanent magnet is moved with respect to the Hall probe are another alternative to potentiometers that is sometimes used. The inventors hereof have found that Hall probes that are able to detect precise variations in a magnetic field, such as caused by the movement of a proximate permanent magnet, are expensive and require operating voltages higher than is commonly used for a microcontroller-based musical effect. Hall probes can also be sensitive to other nearby magnetic fields which is a problem when multiple controllers are being used in proximity to one another. Moreover, magnetic sensors which detect the proximity of a permanent magnet can only be used in indirect control schemes. In electronic musical instrument applications, an alternative low-voltage, accurate and inexpensive position sensor that has the flexibility to be used in both direct and indirect control schemes is highly desirable.
Inductive proximity sensors show promise as an alternative to potentiometers. However, they are not currently used in the electronic musical industry because those that are commercially available are expensive and have technical limitations in this field of application. Commercially available inductive proximity sensors use an inductive coil and a ferrite core material to detect the presence of a nearby metallic object and this metallic object needs to be close to the inductive coil for accurate position measurements to be made, or the inductive coil needs to be prohibitively large. Moreover, this type of inductive proximity sensor will interact with others of the same type if used nearby because of their sensitivity to metallic objects; the metallic objects from the different sensors cannot be measured independently. Current inductive proximity sensors are clearly not suitable for use in electronic musical instrument applications, and an alternative solution is required.
We will describe a solution that improves the robustness of the controller so that it is substantially unaffected by dirt and moisture contamination, is not affected by the levels of shock and vibration experienced during normal use and does not wear out, that is accurate, that is inexpensive and that can be manufactured reliably and reproducibly. For maximum flexibility the new solution needs to be capable of providing accurate position sensing over a range of distances, and be capable of being operated in a multi-sensor environment.
Background prior art can be found in GB2,320,125A; U.S. Pat. No. 4,580,478 and U.S. Pat. No. 4,838,139. Each of these documents describes an arrangement in which the inductance of the active coil is changed to provide a corresponding change in resonant frequency, which is then detected. WO2011/128698 was published after the priority date of the present application, albeit it has an earlier priority date. U.S. Publication No. 2002/0140419, teaches the use of a particular circuit configuration in which the transmitted and received magnetic fields may have the same frequency, teaching the skilled person to employ ‘anti-dazzle means’ to distinguish between the transmitted and received fields. It is desirable to improve upon this approach.